A type of such control apparatuses set forth above is designed to carry out current feedback control to thereby adjust an actual value of at least one controlled variable of a rotary machine to a request value thereof. A typical control apparatus of this type operates in a PWM (Pulse Width Modulation) control mode for switching elements of an inverter as an example of power converters.
The control apparatus for a three-phase motor operates in the PWM control mode to calculate a substantially sinusoidal command voltage for each phase winding of the three-phase motor; this command voltage is required to match an actual current flowing through each phase winding and fed back therefrom with a desired periodic command current.
The control apparatus operates in the PWM control mode to compare the sinusoidal command voltage for each phase winding with a triangular (or saw-tooth) carrier wave. Based on the result of the comparison, the control apparatus operates in the PWM mode to individually switch on and off each of bridge-configured switching elements of an inverter based on the result of the comparison. This modulates an input DC voltage to the inverter into an AC (Alternating Current) voltage to be applied to each phase winding of the rotary machine.
Adjustment of the on and off durations, that is, the duty (duty cycle) of each of the bridge-configured switching elements by the control apparatus matches the AC voltage to be applied to each phase winding with the command voltage therefor. This matches the actual current flowing through each phase winding to a desired periodic command current. The actual current flowing through each phase winding works to generate, as the at least one control variable, a torque corresponding to the desired command current for each phase winding.
The PWM control mode for a three-phase motor needs to increase the command voltage in a higher velocity range of the three-phase motor. The bridge-configured inverter limits an upper limit of the amplitude of the command voltage to substantially the half of the input DC voltage to the inverter. This is because the substantial half of the input DC voltage to the inverter is applied to each phase winding.
Thus, when the command voltage increases in amplitude to be greater than the half of the inverter input DC voltage, an actual output voltage of the inverter cannot be matched with the command voltage.
Thus, in a higher velocity range of a three-phase motor, using a single-pulse control mode in place of the PWM control mode has been implemented. For example, Japanese Patent Application Publications No. 2002-223590 and 2005-218299 disclose control apparatuses operating in the single-pulse control mode.
A control apparatus operates in the single-pulse control mode in a higher velocity range of the three-phase motor to individually switch on and off each of the switching elements of the inverter such that the on and off cycle of each of the switching elements is substantially matched with the period of the periodic command current; this period corresponds to an electric angle of 2π radians.
The control apparatus that operates in the single-pulse control mode in a higher velocity range of the three-phase motor provides a voltage utilization factor greater than that obtained when it operates in the PWM control mode in the higher velocity range. The voltage utilization factor is a ratio of an RMS value of a line-to-line voltage of the three-phase motor to an inverter input voltage.
However, the single-pulse control mode abruptly, that is, discontinuously increases the voltage utilization factor from the value obtained at the moment when the amplitude of the command voltage for the PWM control mode reaches the half of the input DC voltage to the inverter.
An additional control method for continuously shifting inverter control from the PWM control mode to the single-pulse control mode is disclosed in Japanese Patent Application Publication No. H09-047100.
The method disclosed in the Patent Publication No. H09-047100 is designed to, when the amplitude of the command voltage for the PWM control mode reaches the half of the inverter input DC voltage, use a pattern of periodic repetitive pulses stored in a ROM and a phase of a vector of the command voltage in a d-q coordinate system. The d-axis of the d-q coordinate system is in line with a rotor N pole center of a three-phase motor, and the q-axis thereof has a phase of π/2 radian electric angle leading with respect to a corresponding d-axis during rotation of the three-phase motor.
The method is also designed to switch on and off each of the bridge-configured switching elements in accordance with the pattern of periodic repetitive pulses stored in the ROM.
Specifically, the method is designed to, when the norm of a vector of the command voltage is equal to or greater than a preset value, shift the control mode for the inverter from the PWM control mode to the single-pulse control mode to thereby switch on and off the bridge-configured switching elements in accordance with the pattern of the periodic repetitive pulses stored in the ROM.
This makes possible that the voltage utilization factor obtained at the moment when the amplitude of the command voltage for the PWM control mode substantially reaches the half of the inverter input DC voltage is continuously shifted to the voltage utilization factor obtained using the single pulse control mode.
Under the inverter being driven based on the periodic repetitive pulses stored in the ROM, when the request value of the at least one control variable, such as an output torque of the three-phase motor and/or a rotational speed thereof is reduced with reduction in the voltage utilization factor, the norm of the vector of the command voltage is reduced below the preset value. At that time, the method shifts the control mode for the inverter from the single-pulse control mode to the PWM control mode.
However, at the moment when the control mode is shifted from the single-pulse control mode to the PWM control mode, a voltage required for the three-phase motor to create a torque generated immediately before the shift may not be obtained in the PWM control mode. When the voltage required for the three-phase motor to create the torque generated immediately before the shift is not obtained in the PWM control mode, the shift may cause the generated torque by the three-phase motor to be suddenly changed. This may make it difficult to maintain, at a high level, the performance of the current feedback control when the control of the inverter is shifted from the single-pulse control mode to the PWM control mode.